Method for making semiconductive piezoelectric ceramic transducers



E'cTRIc CERAMIC TRANSDUCERS Filed Jan. 12. 1968 YOSl-IIHIRO MATSLIO m'rosn-n mn'rsumd'm. EISUKE KUROKAWA.

HIROMU SASAKI mu: SHIGERU HAYAKAWA INVENTORS BY NM .Lm

ATTORNEYS 3,506,575 METHOD FOR MAKING SEMICONDUCTIVE PIEZOELECTRIC CERAMIC TRANSDUCERS YoshihiroMatsuo, Hitoshi Matsumoto, Eisuke Kurokawa,

HiromuSasaki, and Shigeru Hayakawa, Osaka, Japan, assiguoi's to Matsushita Electric Industrial Co., Ltd., Osaka, Japan Filed Jan. 12, 1968, SEL'NO. 697,462 Claims priority, application Japan, Mar. 14, 1967,

42/ 16,821; Mar. 23, 1967, 42/ 18,574

Int. Cl. C04b 35/00 U.S. -Cl. 252-623 4 Claims ABSTRACT OF THE DISCLOSURE semiconductive material which is characterized by low specific resistivity and superior electromechanical transducing properties is made by firing PbO, MgO, Nb O TiO and ZrO in an inert or reducing atmosphere and corresponds to the formula x+y+z being equal to 1. NiO is optionally included. The material is useful in making e.g. a phonograph pick up for a transistorized amplifier with low input-impedance.

The electronic industries have recently required an electromechanical transducer material having a low specific resistivity, since various electronic equipments have been transistorized. For example, a photograph pickup for a transistorized amplifier needs an electromechanical transducer element having a specific resistivity as low as to 10 ohm-cm, because a transistorized amplifier has a low input-impedance. There has heretofore been known no ceramic electromechanical transducer material applicable to such a phonograph pickup for transistorized amplifiers.

At the present time, the phonograph pickup element for transistorized amplifiers is composed of semiconductive materials having a piezoresistive property. As a piezoresistive material, germanium or silicon single crystals of N-type or P-type semiconductor are commonly used. However, these materials show a small piezoresistive coefficient and, therefore, they exhibit a low output voltage when used in the phonograph pickup elements.

On the other hand, piezoelectric ceramic materials such as lead titanate-zirconate Pb(Ti,Zr)O barium titanate, BaTiO and lead magnesium niobate-titanate-zirconate, Pb(Mg Nb ,Ti,Zr)O are widely used as electromechanical transducer material for phonograph pickup elements. These piezoeletric materials show superior electromechanical transducer properties and high output voltage when operated as a phonograph pickup element. However, they show a higher specific resistivity than the input-impedance of the transistorized amplifier.

The applied voltage to the input of the amplifier by the piezoelectric output voltage is substantially lowered United States Patent 0 ice due to the higher specific resistivity. Thus, the conventional piezoelectric ceramics as electromechanical transducer materials are not suitable for application to transistorized amplifiers.

It is well known that barium titanate becomes semiconductive when incorporated with a small amount of alkaline rare earth metal oxide or fired in a reducing gas atmosphere. However, these semiconductive barium titanate ceramic materials exhibit no piezoelectric properties and, therefore, they are not used as an electromechanical transducer material.

An object of the present invention is to provide a semiconductive piezoelectric ceramic material having a low specific resistivity and superior electromechanical transducing properties.

Another object of the present invention is to provide a method for making said material with use of atmospheric firing.

A further object of the present invention is to provide a method for making an electromechanical transducer comprising said material.

A still further object of the present invention is to increase the electromechanical coupling coeflicient of said material modified by addition of nickel oxide.

These and other objects of the present invention will be apparent upon consideration of the following description taken together with the accompanying drawing in which the single figure is a perspective View of a transducer comprising a ceramic body according to the present invention.

Before proceeding with a detailed description of the semiconductive piezoelectric ceramic materials contemplated by the invention, their application is electromechanical transducers will be described with reference to the aforesaid drawing wherein reference character 10 designates, as a Whole, an electromechanical transducer having, as its active element, a preferably disc shaped body 11 of semiconductive piezoelectric ceramic material according to the present invention.

Body 11 is electrically polarized, in a manner hereinafter set forth, and is provided with a pair of electrodes 12 and 13, applied in a suitable and per se conventional manner, on two opposite surfaces thereof. Lead wires 15 and 16 are attached to the electrodes 12 and 13 respectively by means of solder 14. When the ceramic body is subjected to shock, vibration, of other mechanical stress, electrical output generated can be taken from the lead wires 15 and 16.

The scope of the present invention will be understood more clearly upon consideration of the following description with reference to examples according to the invention.

The raw materials, PbO, MgO, Nb O TiO and ZrO and with or without NiO are used in amounts based on the compositions given in Table I where the compositions are expressed by the chemical formula (x+y+z=1). They are intimately mixed in a ball-mill using an appropriate amount of water. After mixing and drying, the mixture is pressed into desired forms, for example, pellets of 50 mm. diameter and 30 mm. thickness under any suitable pressure, for example, 300 kg. per cmfi. The pressed pellets are calcined at any suitable temperature, for example, at 850 C. for 2 hours in air. The calcined pellets are crushed by ball-milling. After crushing and drying, small amount of any suitable binder such as polyvinyl alcohol (PVA) is added to the calcined powders. The PVA-added powders are pressed into desired form, for example, pellets of 20 mm. diameter and 2 mm. thickness under any suitable pressure, for example, 700 k. per cm.

The pressed pellets are fired in an oxygen-deficient gas atmosphere consisting of flowing nitrogen, argon, hydrogen, or a mixture thereof at 1100 to 1350 C. for a time period of minutes to 5 hours in accordance with the present invention. The firing is carried out under any suitable condition, for example, at the heat-up and cooling rate of 200 C. per hour. During the heat-up, Soaking and cooling process, the atmosphere surrounding the pellets is established by flowing an oxygen-deficient gas selected from nitrogen, argon, hydrogen, or a mixture thereof. By oxygen-deficient atmosphere is meant an atmosphere containing substantially less oxygen than air under normal atmospheric pressure. The flowing rate ranges from to 200 milliliters per minute. The compositions and the firing atmospheres are given in Table I.

Thus obtained solid solution ceramic body of the composition Pb (Mg Nb Ti Zr O with or without NiO is polished down to 1 mm. in thickness and electroded by silver paint. The silver electroded ceramic body is polar- TABLE II Radial coupling Specific coeflicient Dielectric Dielectric resistivity (percent) constant loss factor (ohm-cm) 10.1 500 0. 05 7X10 15. 8 2, 000 0. ()9 9X10 20. 4 4, 700 0. 12 5x10 31. 7 7, 900 0. 23 7X10 29. 5 7, 300 0. 22 2x10 32. 3 6,200 0.15 3 10 30. 2 8, 500 0.25 1 10 31. 5 6, 0.28 2X10 28. 5 7, 500 0. 8X10 40.0 4, 600 0.38 9X10 43. 5 4, 800 0.23 1X10 41.0 13,000 0. 47 1X19 45. 9 300 0.20 5x10 42. 1 12, 090 0.44 2x10 35. 6 ,100 0. 35 1X10 33. 1 8, 700 0. 1 10 29. 0 5, 200 0.39 4X10 58. 5 1, 900 0. 025 4x10 57. 8 3, 600 0.15 1X10 45.6 12, 000 0.39 3X10 39. 1 18, 090 0. 2X10 31. 5 ,000 0. 59 2X10 58. 5 1, 900 0.035 7X10 65.0 4, 000 0. 10 1X10 45. 0 11, 000 0. 35 8x10 42.0 21, 000 0.58 1X10 55. 2 4,100 0.18 7X10 35.0 3, 200 0.35 9X10 28. 0 6, 800 0. 42 6X10 24.0 9,200 0.54 2X10 39. 0 3,100 0.33 1x10 41.0 4, 000 0. 37 5X10" 32. 0 5, 200 0. 37 1 10 39. 0 8,700 0. 51 3X10 30. 5 4, 900 0. 54 1X10 27. 0 4, 100 0. 13 2X10 49. 0 900 0. 018 1X10" 48.0 880 0.018 2X10 TABLE I Compositions Addition Firing atmosphere amount Sample PbMgi/sNbz/aOa PbTiO; PbZrO; (wt. Flowing rate N o. x y z Additives percent) Flowing gas (ml./m1n.)

1. 00 0. 00 0. 00 0 95% N2+5% Hz--- 20 0.90 0. 10 0. 00 0 95% Nz+5% H2.-- 20 0.80 0. 20 0. 00 0 95% Nz+5% 2.-- 20 0.79 0. 20 0. 01 0 95% N2+5% H2-" 20 0. 79 0. 20 0. 01 0 80% Nz+20% H2 20 0. 69 0. 30 0. 01 0 95% Nz+5% H2--- 20 0.69 0.30 0. 01 0 80% Nz+20% Hz. 20 0. 58 0.41 0.01 0 95% Nz+5% Hz--- 20 0. 58 0.41 0. 01 0 80% Nz+20% H;| 20 0.50 0. 375 0.125 0 95% N2+5% Hz--. 20 0.50 0. 375 0.125 0 N t- 20 0.50 0.375 0. 125 0 95% N2+5% H2..- 20 0.50 0. 375 0.125 0 N1 20 0.50 0.375 0.125 0 95% Nz+5% Hz... 20 0.40 0. 0.01 0 95% N +5% Hz..- 20 0.49 0. 30 0.21 0 95% N 2+5% H2.-. 20 0. 4375 0.4375 0. 125 0 N 200 0. 375 0. 375 0. 25 0 20 0. 375 0. 375 0. 25 0 50 0.375 0. 375 0.25 0 N 200 0. 375 0.375 0. 25 0 95% Nn+5% H2.-- 50 0. 375 0. 375 0. 25 0 80% Nz+20% Hz. 20 0. 375 0. 375 0.25 0.3 N; 50 0. 375 0. 375 0. 25 1.0 Na 50 0. 375 0. 375 0. 25 1. 0 95% N2+5% H2... 50 0. 375 0. 375 0.25 l. 0 80% Nz+20% Hz- 20 0. 375 0. 375 0. 25 1. 0 Ar 50 0. 125 0.435 0. 44 0 N2- 200 0. 125 0.435 0.44 0 95% Nz+5% H2... 50 0. 125 0.435 0. 44 0 N2+40% H2- 20 0.125 0.435 0.44 0. 3 N2 200 0. 125 0.435 0.44 1. 0 2 200 0.063 0. 52 0.417 0 95% N 2 5% Hz--. 50 0.063 0. 52 0. 417 0 80% Nz+20% Hz. 20 0. 063 0. 30 0.637 0 80% Nz+20% Hz" 20 0.01 0.46 0. 53 0 80% Nz+20% Hz 20 0. 00 0. 47 0. 53 0 N2 200 0. 00 0.47 0.53 0 60% Nz+40% 20 ized by pulsing of a DC (direct current) field of 40 kv. (kilovolts) per cm. preventing heat generation.

Some compositions investigated do not exhibit high piezoelectricity and many are electromechanically active only to a slight degree. The present invention is concerned only with those compositions exhibiting piezoelectric response of appreciable magnitude. As a matter of convenience, the radial cOupling, Kr (also known as planar coupling Kp and disc coupling, K disc) by the IRE method, of the test body is taken as a measure of piezoelectric activity for the transducer material. The piezoelectric and dielectric properties and specific resistivity of the ceramic 'body investigated are given in Table II.

The specific resistivity in the order of 10 -10 tZ-cm. can be obtained by the atmosphere firing of the composition ceramics listed in the Table III. In particular, the sample 11-1 comprising fired in nitrogen atmosphere at the flowing rate of 20 milliliters per minute gives a specific resistivity of 4X10 ohm-cm, a radial coupling coefiicient of 58.5%, and a dielectric constant of 1,900, and the sample 11-4 comprising Pb(Mg1 3Nb2 3) 375TT 375ZI 25O fired in a gas atmosphere of 95% nitrogen and 5% hydrogen at the flowing rate of 20 milliliters per minute gives 2x10 ohmcm., 39.1%, and 1,800, respectively.

Further, according to the present invention, said semiconductive piezoelectric ceramic materials are improved in radial coupling coefficient by an addition of NiO to said composition, Pb(Mg Nb Ti Zr O The preferable weight percent of said additive is 0.3-1.0 Weight percent of NiO to said composition.

These improvements are summarized in Table IV with reference to the data of Table I and Table II.

where x+y+z:1 and x ranges from 0.063 to 0.79, y from 0.2 to 0.52, and z from 0.01 to 0.637, which comprises firing a corresponding mixture of PbO, MgO, N-b O TiO and Zr at a temperature of 1100 to 1350 C. for a time period of 10 minutes to hours in a reducing and/or inert atmosphere.

2. A method of preparing a semiconductive piezoelectric ceramic material consisting essentially of where x+y+z:1 and x ranges from 0.063 to 0.79, y from 0.2 to 0.52, and z from 0.01 to 0.637, which comprises firing a corresponding mixture of PbO, MgO, Nb O TiO and ZrO at a temperature of 1100 to 1350 C. for a time period of minutes to 5 hours in a gaseous atmosphere consisting essentially of a member selected from the group consisting of nitrogen, argon, hydrogen and mixtures thereof.

3. A method according to claim 1 of preparing a semiconductive piezoelectric ceramic body, the starting mixturefurther containing NiO ranging from 0.3 to 1.0 percent by Weight.

4. A method according to claim 3 in which x ranges from 0.125 to 0.50, y from 0.375 to 0.435, z from 0.125 to 0.44.

TABLE IV Firing Atmosphere Radial coupling Flowing rate N iO addition coefficient Compositions Flowing gas (mL/min.) (wt. percent) (percent) gtls bzla)o.12s lo.4a5 1n.440a N2 200 0 35. 0 gi/s 2/s)o.r25 }o.la5 o.440a N2 200 0.3 39. 0 P (Mgila rls)o.i25T}o.ra5Zro.44Oa 200 1. 0 41. 0 g1/a 2/s)o.s75 o.a'15 o.250s.. 50 0 57. 8 1/sN 2/3)0-875 0.375 n.250a- 50 0. 3 58. 5 Pb g1I3 2/3)ll-375 0.375 I'0.25 a- 50 1. 0 65. 0 E1/aNb2/a) u.s lu.s'l5 0.25 s 50 0 39. 1 us mM.s7s }u.s75 'o.250a 95% Nari-5% H2. 50 1. 0 45. O gi/a b2/a)o.ara ioms lmsoa 80% N2+20% Ht. 0 31. 5 b( r/a 2/:)0 31a 0.a75 o.2s ari-20% H2 20 1. 0 42. 0 N2 2O 0. 3 43. 5 20 1. U 45. 9 2O 0 40. 0 20 0. 3 41. O 20 1. 0 42. 1

References Cited UNITED STATES PATENTS While there have been described presently preferred 4 illustrative examples of thls Invention, various changes 3,268 53 8/1966 Ouchl et 252-629 and modifications can be made therein without departing from the invention, and it is intended, therefore, to

OTHER REFERENCES Gerson et al., J. Phys. Chem. Solids, vol. 24, pp. 979- 84 (1963).

TOBIAS E. LEVOW, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R. 

